I got the Stay 2 at 1 x CAT 1-4 Cert (max 3) as well. So, 6 stays for this period.
Here are all the T&C that I found for others that want to try to switch:
Stay 2 Get 1 CAT 1-4 Cert (Max 3)
Stay 15 Get 25K or Stay 20 Get 40K
Stay 20 Get 35K or Stay 25 Get 50K
And the reason why someone need to login to the UK site for the CAT 1-5 Cert is because there is no T&C under the US link. Here is the T&C for this one from the UK link:
Stay 2 Get 1 CAT 1-5 Cert (Max 2)
I didn't play with all the language, only US and UK.
Regards.